In dual sweep oscilloscopes having delaying and delayed sweeps, the delaying sweep, hereinafter referred to as main sweep, is triggered either automatically or by some electrical event, and an unblanking gate waveform and a sweep sawtooth waveform are simultaneously produced to respectively turn on the electron beam in a cathode-ray tube and sweep the electron beam across the face of the cathode-ray tube at a linear rate to provide a visual display. The main sweep sawtooth is applied to a trigger pickoff circuit to generate a triggering signal to initiate the delayed sweep.
The trigger pickoff circuit usually includes a comparator having a pair of input terminals to which an adjustable reference voltage level and the linearly rising delaying sweep sawtooth voltage are applied. It is a common practice to utilize a linear 10-turn potentiometer called a Delay Time Multiplier to provide the adjustable reference voltage for the comparator, and the Delay Time Multiplier usually has a readout dial which is calibrated to the overall amplitude of the main sweep sawtooth so that a reading is provided which is indicative of the time difference between the start of the main sweep and the point at which the delayed sweep is triggered, such as is disclosed in U.S. Pat. No. 3,551,733.
When the delayed sweep is initiated, a sweep sawtooth waveform and an unblanking gate waveform are generated in essentially the same manner as for the main sweep to provide a delayed sweep display.
Typical operation of a dual-sweep oscilloscope is as follows: In a main sweep mode, an electrical phenomenon is observed in the waveform display, which the observer wishes to examine in closer detail. An intensified mode is selected whereby the unblanking gates of the main and delayed sweeps are algebraically added together so that the main sweep appears at normal intensity and the relative time position of the delayed sweep appears as a brightened segment or zone upon the main sweep display. By turning the Delay Time Multiplier dial, the observer can move the intensified segment to select the triggering point of the delayed sweep at any point over the length of the main sweep. The length of the intensified segment can be expanded or contracted by changing the sweep rate of the delayed sweep. After the observer has intensified the particular phenomenon he wishes to examine and selected suitable sweep rates, the delayed sweep display mode is then selected and the chosen phenomenon observed thereon.
The accuracy of the time interval measurement of the aformentioned prior art system is dependent upon several variables, for example, mainsweep linearity, Delay Time Multiplier potentiometer linearity, dial resolution and graticule interpretation. The flexibility of such a system is subject to human error and is extremely limited since the time interval measurement can be taken only with respect to the start of the main sweep.
One attempt to increase flexibility of time interval measurements was the two-dot indicator system disclosed in U.S. Pat. No. 3,881,133. This system included two controls, one for moving both dots on the main sweep and the other for controlling the separation between the dots. Thus time interval measurements could be made between any two points of interest on a main sweep display rather than only between the sweep start and a single point of interest.